Improving image sharpness is important in image detail enhancement processing. FIG. 1 shows a system block diagram for a conventional image detail enhancement system 10 using a low pass filter (LPF) 12, which is typically known as “unsharp” filter. An input signal f (representing at least a portion of a video image formed by pixels), is processed and output as a signal g. The signal f is provided to the low pass filter (LPF) 12 to generate an unsharp signal f1. Then the difference between the signals f and f1 is determined in a difference junction/node 14, as a detail (difference) signal, (f−f1). The detail signal is then multiplied by a constant K (K>1) for enhancement in a multiplication junction/node 16, and summed with the signal f in a summing junction/node 18, to generate the output signal g. As such, the relationship between the output signal g and the input signal f can be expressed in the following equation:g=(f−f1)*K+f  (1)
In equation (1), the term (f−f1)*K, is the detail enhancement term, and when there is a non-zero difference between the signals f and f1, there are details which may be enhanced in the detail enhancement process.
However, the images to be enhanced may have different types of characteristics. Some images may contain rich high frequency, indicating many fine details or textures. Some other images may contain less high frequency, and so appear relatively smooth. Therefore, a problem with such a conventional detail enhancement system is that it is very difficult to design a LPF that can properly process different types of images with different characteristics. When the highest frequency of a signal is not much higher than the cutoff frequency of the LPF, the detail signal f−f1 would be too weak to provide noticeable enhancement to the original signal. As a result, images containing less high frequency may be enhanced poorly by conventional systems.
In order to solve this problem, other methods using a few filters have been proposed. For example, in U.S. Pat. No. 5,001,573 ('573 patent) two LPFs are used to obtain two low passed versions of the original signal. The signal from the LPF with a lower cut-off frequency is termed as unsharp signal, U, the signal from the other LPF with a relatively higher cut-off frequency is termed as middle signal, M, and the original signal is denoted as S. According to that '573 patent, image detail enhancement depends on the difference signals S−U, M−U and S−M. The enhanced image signal ES can be expressed in a general form:ES=S+k1(S−M)+k2(S−U)+k3(M−U)  (2)
wherein k1, k2 and k3 are all constant coefficients which may have either positive or negative values. If a constant coefficient takes a positive value, the corresponding difference signal is an enhanced term which is added to the original signal. If a constant coefficient takes a negative value, the corresponding difference signal is a suppressed term which is deducted from the original signal. A constant coefficient may also equal to zero if the corresponding difference signal is not taken into account in the detail enhancement process. The '573 patent also states some conditions about k1, k2 and k3 that should be met for a detail enhancement system so that the high frequency component of the original image is always suppressed to eliminate granular noise while the mid frequency component of the original image is enhanced.
The '573 patent provides an extension of the traditional image detail enhancement system of FIG. 1. However, the invention in the '573 patent is too simplistic for the task of detail enhancement on different types of images available today. For example, with the introduction of digital TV, the quality of TV images may vary significantly. High definition images may have rich high frequency but very low level noise. In this case, the suppression of the high frequency component of the original image is not desirable because it simply degrades the image quality and introduces blurring. In another case, when a standard definition image is scaled up to fit a high definition TV screen, the scaled image may only have very low frequency component because of scaling. Therefore, even the LPF used for obtaining unsharp signal in the '573 patent may have too high a cut-off frequency for such images. As a result, the difference signals, S−U, M−U and S−M may all be too weak to bring any significant enhancement to the original image.
As such, there is a need for a practical image detail enhancement method and apparatus suitable for processing different types of digital images, regardless of their signal frequency range or noise level.